omero.gateway: deprecate BlitzObjectWrapper.getExternalInfo()

[sbesson]

See https://github.com/ome/omero-py/issues/481 and the latest discussion

BlitzObjectWrapper.getDetails().getExternalInfo() should provide the same lazy-loading functionality and return a wrapped ExternalInfo

Consumers who might have adopted the OMERO.py gateway API introduced in https://github.com/ome/omero-py/pull/453 should update their code to use the existing object.getDetails().getExternalInfo() API which is separately being fixed with #482.

This approach is consistent with similar metadata like getDetails().getGroup(), getDetails().getOwner() or getDetails().getPermissions(). Unlike BlitzObjectWrapper.getExternalInfo(), this API will return a wrapped ExternalInfo object allowing to use the convenience methods of the gateway e.g. object.getDetails().getExternalInfo().getLsid()